I suggest you ...

GPU passthrough to Linux guests using open source video drivers

GPU passthrough to Linux guests using proprietary drivers is spotty at best. For example, I can get GPU passthough working with an Ubuntu 12.04 HVM guest using one older Catalyst driver, but not with any newer drivers. Moreover, no matter what Catalyst driver I try, I have had absolutely no luck getting GPU passthough working with an Ubuntu 13.04 HVM guest.

Suggestion: Given the spotty nature of propriety drivers, I hope that the Xen Project will work closely with the developers of open source video drivers to enable GPU passthrough to Linux guests using open source drivers.